A methanol carbonylation process (methanol-acetic acid process) is known as a process for industrially producing acetic acid. With this process, an acetic acid product is produced typically by allowing methanol to react with carbon monoxide in the presence of a catalyst in a reactor, to form acetic acid in a reaction mixture, evaporating the reaction mixture using an evaporator into a vapor phase, and purifying the vapor phase through a light ends column, and subsequently through a dehydration column. Alternatively, the product from the dehydration column is further fed to a subsequent heavy ends column and, in some cases, a subsequent product column to give an acetic acid product.
In the acetic acid production process as above, acetaldehyde, which results from reduction of methyl iodide, is converted into crotonaldehyde by aldol condensation, and causes the acetic acid product to yield a worse potassium permanganate test result (permanganate time). In addition, crotonaldehyde is converted into 2-ethylcrotonaldehyde by aldol condensation with acetaldehyde; and 2-ethylcrotonaldehyde also causes the acetic acid product to yield a worse potassium permanganate test result. However, as compared with 2-ethylcrotonaldehyde, crotonaldehyde more worsens the potassium permanganate test result per mass unit, and, when contained in the acetic acid product, more significantly worsens the quality of the product.
To decrease crotonaldehyde and/or 2-ethylcrotonaldehyde, roughly classified two techniques have been conventionally industrially employed (Patent Literature (PTL) 1 and PTL 2). One is (i) the technique of restraining the formation of crotonaldehyde in a reaction system by removing acetaldehyde, which is by-produced in the reaction system, from methyl iodide in a purification step, and decreasing acetaldehyde in the methyl iodide to be recycled to the reaction system. The other is (ii) the technique of directly decomposing crotonaldehyde oxidatively with ozone, where the crotonaldehyde is contained in a crude acetic acid which is obtained in the middle of a purification step. However, facilities for the acetaldehyde separation and removal, and facilities for the ozone treatment are both expensive. Conventional acetic acid production processes entirely depend on these techniques so as to give an acetic acid product that yields a better potassium permanganate test result, and this leads to increase in installation cost.
The methanol-acetic acid process is known to give alkanes as impurities. The alkanes are impurities which contain 3 or more carbon atoms, and which have higher boiling points as compared with methyl iodide and methyl acetate. The alkanes are mainly saturated or unsaturated hydrocarbons, but may contain an oxygen atom and/or an iodine atom in the molecule. Japanese Unexamined Patent Application Publication (JP-A) No. H04-295445 discloses a technique for the removal of the alkanes. In the technique, of overhead condensates from a light ends column, an organic phase is subjected to distillation in a distillation column (alkane-removing column), to give an overhead product including methyl iodide, methyl acetate, and carbonyl impurities, and bottoms including alkanes, water, and acetic acid; the overhead product is recycled to the reactor or fed to an acetaldehyde-removing column, the bottoms are combined with and extracted with water to give an aqueous phase including acetic acid, and an organic phase including alkanes; the aqueous phase is recycled to the reactor, and the organic phase including the alkanes is discarded as a waste. This literature, however, neither discloses nor indicates how to allow the acetic acid product to yield a better potassium permanganate test result.